This invention relates to an apparatus for heating air that is to be supplied to a blast furnace.
A blast furnace is used for extracting iron from iron ore. Coke and limestone are charged through a hopper in the upper end of the furnace. Heater air is fed into the furnace through a ring of holes located near the base of the furnace. The upflowing hot air burns the coke which forms carbon monoxide that reacts with the iron ore to reduce and liquify the iron. Molten iron gravitates downwardly to the bottom of the furnace where it is periodically removed.
Air supplied to the furnace should be at 1800.degree. F. for proper operation of the furnace. Under conventional practice the air is heated by passing it through large stoves containing refractory checkerwork. One such stove is shown in U.S. Pat. No. 3,122,359 to J. McDonald. Operation of such stoves is cyclic in nature. During part of the cycle, carbon monoxide is fed from the blast furnace to burners in the stove. The burners heat the checkerwork. In the other part of the cycle, the burners are shut down, and the air for blast furnace operation is passed through the checkerwork to extract heat previously generated in the bricks by operation of the burners. The heated air is passed from the stove to the blast furnace.
There are some disadvantages with the conventional system for heating air being supplied to a blast furnace. For example, two or three stoves must be provided for a single blast furnace (because each stove is delivering heated air to the furnace only part of the time). Also, the air coming out of the stove is usually not at a uniform temperature, due to flow variances across the checkerwork, and also due to the fact that the bricks (checkerwork) have a lesser air heating effect as they become cooler during the course of the air heating cycle.
The present invention contemplates an air heating apparatus that includes several air heater devices arranged in a series-flow relation. Each device raises the air temperature a given amount (but not necessarily the same amount for each device). The total heating effect is the sum of the heating actions of the various heater devices in the system.
A typical system comprises twelve heater devices, each heater device being effective to raise the air temperature so that the total increase is about 1200.degree. F.
Each heater device preferably includes a tube-shell heat exchanger having three or more forced draft burner units extending radially for directing the burner flames through the shell and onto the tubes. The air is blown through the tubes, such that the heated tubes heat the flowing air. An air expansion space is formed at the outlet end of each air heater device so that a turbulent mixing of the heated air occurs each time the air leaves a given set of tubes. This mixing action tends to promote a uniform air temperature, to minimize any undesired hot or cold spots that might otherwise result due to uneven heating of the tubes by the burner.
Each heating device is independently controlled to obtain a desired air outlet temperature. The air is heated in small steps, such that effects of under heating or over heating in any one step are compensated for (corrected) by the next heating step.
A primary advantage of the heating apparatus is that it is continuous (not cyclic). Therefore it can supply the entire air heating requirements of a given blast furnace, without any down time for regeneration or reconditioning.